Since the nineteenth century, it has been well known by physicians that a medical diagnosis can be determined by listening to the sounds emanating from a patient's internal organs either in operation or as a result of a physician's rapping on the patient's chest with his fingers. This method of medical diagnosis is known as auscultation. In order to amplify these sounds, the stethoscope was invented in 1816 by the French physician Rene Laennec and has since proven to be an invaluable tool in noninvasive medical diagnosis.
Auscultation is considered by many cardiological authorities to be the most sensitive test of the functional integrity of the heart. Not infrequently, murmurs or changes in the heart sounds are the only concrete indication of heart disease. The ability to distinguish between what is normal and what is abnormal is particularly difficult in many borderline cases.
Cardiovascular sounds, however, include components that have frequencies and intensities which are not within the auditory range of the human ear. As a result, only a small fraction of these sounds or acoustic vibrations can be heard using a conventional stethoscope. There have been attempts to solve the problem of low signal levels by the use of electronic stethoscopes using microphones and signal amplifiers. However, the use of such equipment is limited in frequency due to inherent insensitivity o the human ear, as well as the earphones used in these electronic stethoscopes.
Electronic stethoscopes have been devised which enable a user to hear heart sounds more easily. For example, U.S. Pat. No. 4,528,689 discloses an electronic stethoscope which provides a signal which is similar to but not exactly the same as a slowed down version of the original sound by producing an output signal which is composed of replicated sets of cycles of the original sound. The output signal, however, does not maintain the tonal relationships of the original sounds, is not a truly frequency shifted version of the original sound, and is merely the original signal periodically reproduced so as effectively to have a longer duration.
Other electronic stethoscopes have been devised which shift low frequency signals into a frequency within the ear's audible range, however, such prior art devices all have their limitations.
For example, U.S. Pat. No. 3,562,428 discloses an electronic stethoscope which modulates the detected sound so that low frequency signals are shifted into the audible frequency range. The quality of the sound generated by this stethoscope is substantially different from the usual sounds heard by the physician since a carrier signal is added to the originally detected sound whereby the detected sound is merely shifted into a higher frequency range without maintaining the same tonal relationships as in the originally detected sound.
U.S. Pat. No. 4,220,160 is directed to a device for transposing heart sounds into the audible frequency range by processing certain predetermined frequency components of the detected sound to obtain an output signal which consists of the sum and difference of the frequency components and a carrier frequency signal. The output signal is then filtered to remove the difference between the frequency components.
U.S. Pat. No. 4,594,731 discloses an electronic stethoscope in which the detected heart sounds are frequency multiplied to transpose the sounds to a more favorable position within the auditory range. The signal is input to a frequency doubling circuit consisting of a full wave rectifier, bandpass filter and a wave shaping circuit.
The main disadvantage to these prior art electronic stethoscopes is that the tonal relationships in the sounds generated by the electronic stethoscopes are different from the tonal relationships in the original sounds and thus do not blend in the same way. For example, a group of tones that are separated by octaves (Two signals are said to be separated by an octave if the ratio of their frequencies is equal to two.) in the original sound (10 Hz, 20 Hz, and 40 Hz) would no longer be separated by octaves in the signal after processing by the electronic stethoscopes because the detected signal is merely shifted to a different frequency range (110 Hz, 120 Hz, and 140 Hz). Thus the sounds generated by such electronic stethoscopes are not similar to the original sounds, do not combine with each other in the same way as the original sounds, and are unrecognizable to experienced physicians trained to recognize the sounds of a conventional stethoscope. Similar problems arise using the electronic stethoscopes in which the frequencies of the sound are simply multiplied by a constant value. It is important that not only the relative frequency relationships between signal components be maintained, but also the relative phase relationships as well as the amplitude of the signal components. The frequency multiplication circuitry described in U.S. Pat. No. 4,598,731 not only does not maintain the relative phase relationships of the frequency components, as well as the amplitudes but also generates new unwanted frequency components. This combination changes the shape of the waveform and the melody of the heart sounds so that they are no longer recognizable as a transposed version of the original heart sounds.